. PROJECT SUMMARY/ABSTRACT The immunopathogenesis of Crohn's Disease (CD) is characterized by defective immunoregulation of innate and adaptive immune responses toward intestinal microflora, leading to uncontrolled inflammatory T helper cell (Th)1 and Th17 responses via a yet unknown mechanism. Although the proinflammatory activation of professional antigen presenting cells (APCs) thorough MyD88-dependent toll like receptors (TLRs) has been proposed as the mechanism, abrogation of pathogenic responses of professional APCs leads only to a strong improvement, but not full restoration, of the intestinal balance in animal models of CD. This incomplete response raises the question: what role do non-professional APCs such as CD90+ stromal cells, an abundant cell type in the gut lamina propria, play in these processes? Our objective is to identify how TLR-dependent and independent signaling processes in human intestinal colonic mucosal CD90+ stromal cells (myofibroblasts/fibroblasts, CMFs) are involved in the regulation of Th1/Th17 intestinal balance, and determine how these processes are disrupted in CD. Our central hypothesis is that a switch in the CD90+ (myo)fibroblast function from immunosuppressive (IL-6lowPD-L1+) toward inflammatory (IL-6highPD-L1low) is a key process in the persistence of the Th1/Th17 responses in CD. The rationale for this hypothesis is that normal (N-) CD90+ CMFs suppress undesirable Th1 type acute inflammation mediated via PD-L1 (Programmed cell death 1 ligand 1) and stimulate the generation of CD4+ regulatory T cells. In contrast, our data suggest that CD-CMFs display a fundamentally altered phenotype: they have low expression of Th1 suppressive molecule PD-L1, reduced ability to generate CD4+ regulatory T cells and upregulated basal and TLR inducible IL-6 secretion, aTh17 promoting cytokine. Our initial in vivo data with use of fibroblast-specific MyD88 conditional knockout mice also support the importance of CMFs in the regulation of the Th1/Th17 responses. We will test our hypothesis by addressing the following specific aims: (1) Define role of the proinflammatory activation of human CD-CMFs in the regulation of Th1/Th17 responses; (2) Elucidate mechanism(s) responsible for the inflammatory (PD- L1lowIL-6high) activation of CD-CMFs; (3) Determine the pathophysiological role CMF inflammatory activation to the development of CD murine colitis. This project is highly significant to the integrative global research priorities in IBD: it will elucidate previously unexplored interactions between innate and adaptive immune components contributing to the initiation and progression of CD. We expect to provide key mechanistic insights into the role of CMFs in regulation of the Th1/Th17 responses during CD immunopathogenesis. As a translational project, we expect to identify new therapeutic targets for effective treatments of CD.